This invention relates to a voltage controlled type electronic musical instrument using a voltage controlled type variable frequency oscillator as a tone generator, and more particularly to a keyboard circuit for generating a pitch determining voltage signal which is supplied to a voltage controlled oscillator.
Conventionally known is an electronic musical instrument utilizing a voltage controlled oscillator (hereinafter referred to as VCO) as a tone generator. The use of VCO contributes greatly to the simplification of a tone generating circuitry for a monophonic electronic musical instrument. Such an electronic musical instrument is usually constructed as shown in FIG. 1 so that a pitch determining voltage signal having a voltage corresponding to the note of a key operated on a keyboard is derived from a keyboard circuit 1 and then coupled to VCO 2. VCO 2 generates a tone signal having a tone pitch determined by the pitch determining voltage signal. The tone signal is fed to a tone coloring filter 3 where a desired tone color is imparted to the tone signal. The output signal of the tone coloring filter 3 is reproduced by a reproducing system not shown.
An electronic musical instrument as shown in FIG. 2 has also been developed. That is, a voltage controlled lowpass filter 12 (hereinafter referred to as VCF) and voltage controlled amplifier 13 (hereinafter referred to as VCA) are serially connected at the output side of VCO 11. VCO 11 generates a tone signal in response to the pitch determining voltage signal from a keyboard circuit 14 and the tone signal is then applied to VCF 12. VCF 12 is also coupled to the keyboard circuit 13 so that the cutoff frequency thereof varies in accordance with the voltage value of the pitch determining voltage signal.
The keyboard circuit 14 generates, in addition to the pitch determining voltage signal, a trigger signal resulting from the actuation of a key on the keyboard. The trigger signal is supplied to control waveform generators 15, 16 and 17. The respective control waveform generator is responsive to the trigger signal to produce a control waveform whose voltage value varies as a function of time. The output voltages of the control waveform generators 15, 16 and 17 are coupled to VCO 11, VCF 12 and VCA 13, respectively. The oscillation frequency of VCO 11, cutoff frequency of VCF 12 and gain of VCA 13 are transiently controlled in accordance with the waveforms of the control voltages, thereby providing particular sound effects. Parameter controlling voltage generators or potentiometers 18 which are manipulated by a player are provided for controlling various parameters of control waveforms from the control waveform generators 15, 16 and 17 so as to vary the waveforms. By so doing, sound effects can be freely controlled by the intention of the player.
The output of VCA 13 is supplied to a reproducing system including a power amplifier 19 and a loudspeaker 20.
Where VCO as shown in FIGS. 1 and 2 is used, it is required that the voltage values of the pitch determining voltage signals from the keyboard circuit exponentially vary in the order of the tone pitch in case of the equally tempered musical scale. Since in the equally tempered musical scale the frequency ratio of adjacent two tones is 2.sup.1/12, it is required to generate voltage signals in accordance with the frequency relation of the equally tempered musical scale. That is, the voltage ratio of the pitch determining voltage signals corresponding to adjacent two keys on the keyboard is required to be 2.sup.1/12.
FIG. 3 shows a prior art keyboard circuit for such a use. In this figure, single-pole double-throw key switches S.sub.1, S.sub.2, S.sub.3, . . . S.sub.n.sub.-1 and S.sub.n are serially connected, in the sequence of the tone pitch, with respect to their normally closed fixed contacts and movable contacts. The normally open fixed contacts of the key switches S.sub.1, S.sub.2, . . . S.sub.n.sub.-1 and S.sub.n are each connected to a junction or voltage dividing point of a plurality of equally valued resistors RO serially connected across a DC voltage source E. The pitch determining voltage signal is derived from the movable contact of the key switch S.sub.n situated on the extreme right-hand side of the key switch arrangement i.e. driven by the highest note key on the keyboard. That is, the key switch arrangement of FIG. 3 is of a higher tone preference type. With this arrangement, however, the voltage of the pitch determining voltage signal varies merely in a linear fashion with respect to the notes of the keys. Consequently, it is necessary to provide an exponential amplifier 30 at the output of the key switch S.sub.n to convert the linear voltage variation into the exponential voltage variation.
Since with the arrangement of FIG. 3 the pitch determining voltage signal is derived through the series-connected key switches, it is necessary to use highly reliable key switches. Otherwise, drawbacks of key switch contacts, for example, contact resistance are accumulated to affect the voltage level of a pitch determining voltage signal. One-pole double-throw key switch is unpreferable in the reliability due to its complicated construction.
FIG. 4 shows another prior art keyboard circuit arrangement. Key switches S.sub.1, S.sub.2, . . . S.sub.n.sub.-1 and S.sub.n having the same contact structure as those shown in FIG. 3 are connected in like manner. In a voltage dividing network, a plurality of first resistors r having an equal value are connected in series with each other, and the series connection of the first resistors r has one end, i.e. highest tone side, connected to a DC source E and the other end, i.e. lowest tone side, grounded through a second resistor R.sub.1. A plurality of third resistors R having an equal value are connected between corresponding junctions between the series-connected adjacent two first resistors r and ground. Each dividing point is connected to a normally open fixed contact of the corresponding key switch.
If the following relations are established between the resistors r, R.sub.1 and R, pitch determining voltage signals exponentially varied with respect to notes as shown in FIG. 6 are obtained from the output terminal O without using an exponential amplifier. ##EQU1##
The features of the keyboard circuit as shown in FIG. 4 reside in that the voltage dividing network is connected directly to the DC voltage source and that upon the key actuation the pitch determining voltage signal is derived through one or more series-connected key switches from the voltage dividing point. The reason why the key switches having the complicated contact structure are used in the arrangement shown in FIG. 4 is to assure deriving from the output terminal a pitch determining voltage signal perfectly corresponding to the highest or lowest note of actuated keys.